如何向 jsp:useBean 引用的 ArrayList 添加值?

问题描述

在 JSP/JSTL 中，如何为 class="java.util.ArrayList" 的 usebean 设置值.

In JSP/JSTL, how can I set values for a usebean of class="java.util.ArrayList".

如果我尝试使用 c:set 属性或值，我会收到以下错误:javax.servlet.jsp.JspTagException: Invalid property in : "null"

If I try using c:set property or value, I get the following error:javax.servlet.jsp.JspTagException: Invalid property in : "null"

推荐答案

这不是直接可能的.有 和 标签，它们允许您通过 setter 方法在完整的 javabean 中设置属性.但是，List 接口没有 setter，只有一个 add() 方法.

That isn't directly possible. There are the <c:set> and <jsp:setProperty> tags which allows you to set properties in a fullworthy javabean through a setter method. However, the List interface doesn't have a setter, just an add() method.

解决方法是将列表包装在真正的 javabean 中，如下所示:

A workaround would be to wrap the list in a real javabean like so:

public class ListBean {

    private List<Object> list = new ArrayList<Object>();

    public void setChild(Object object) {
        list.add(object);
    }

    public List<Object> getList() {
        return list;
    }
}

并设置它

<jsp:useBean id="listBean" class="com.example.ListBean" scope="request" />
<jsp:setProperty name="listBean" property="child" value="foo" />
<jsp:setProperty name="listBean" property="child" value="bar" />
<jsp:setProperty name="listBean" property="child" value="waa" />

但这没什么意义.如何正确解决它取决于唯一的功能需求.如果您想在 GET 请求时保留一些 List，那么您应该使用预处理 servlet.创建一个 servlet，它在 doGet() 方法中执行以下操作:

But that makes little sense. How to solve it rightly depends on the sole functional requirement. If you want to preserve some List upon a GET request, then you should be using a preprocessing servlet. Create a servlet which does the following in doGet() method:

List<String> list = Arrays.asList("foo", "bar", "waa");
request.setAttribute("list", list);
request.getRequestDispatcher("/WEB-INF/page.jsp").forward(request, response);

当您通过 URL 调用 servlet 时，该列表位于转发的 JSP 中

When you invoke the servlet by its URL, then the list is in the forwarded JSP available by

${list}

不需要老式的标签.在 servlet 中，您可以按照通常的方式自由地编写 Java 代码.通过这种方式，您可以仅将 JSP 用于纯粹的演示，而无需通过 标签吞噬/破解一些预处理逻辑.

without the need for old fashioned <jsp:useBean> tags. In a servlet you've all freedom to write Java code the usual way. This way you can use JSP for pure presentation only without the need to gobble/hack some preprocessing logic by <jsp:useBean> tags.

• 我们的 servlet wiki 页面

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